Blog 10

In the interest of adding some a quick enhancement to the graphics of our game, I looked through some additional effects

Fog

Fog is the natural phenomenon in which water in the air obscures light, the further away, the more fog obscures the light reaching the viewer. Fog can be useful as a way to hide geometry and create a sense of depth and atmosphere. In old OpenGL fog can be achieved by just enabling it and setting various attributes. It is performed at the end of the pipeline, affecting the final pixels. With the use of shaders, variance and customization is available to create more interesting varieties of fog. To create a fog effect, the distance of a pixel to the camera is taken into account, the further away, the more foggy; this is akin to depth of field. There are many ways to create fog such as by using height or even particles so that the fog can be affected by light and shadow.

A simple implementation:

vec3 fogColour = vec3(0.722, 0.961, 0.949);
float z = gl_FragCoord.z / gl_FragCoord.w;
float density = 0.001;
float fog = exp2(-density * density * z * z);
fog = clamp(fog, 0.0, 1.0);

fragColour.rgb = mix(fogColour, diffuse, fog);

Light fog on distant ocean

For more advanced fog effects, the location of the light source can be taken into account to create regions in which fog is illuminated the closer it is to the light source – blend between the color of the fog and the color of light using the dot product of the light vector and the distance to the pixel (similar to diffuse lighting), do this on top of regular fog calculations.

God Rays, also known as light scattering and crepuscular rays is the effect of rays of light hitting small surfaces such as particles or fog and scattering. The resulting scatter of light creates a visible streak of light. God rays are a post-processing effect. Firstly, a vector is created from the light source to the screen space specific pixel in the fragment shader. This vector is then sampled along with different values of decay and weighing to determine its color.

To continue from last week’s blog, I’ve managed to implement a basic particle system rendered using the geometry shader. The important element on top of using it for smoke, clouds and ash in game is that it serves the purpose of creating in-game messages such as score updates and player instructions.

Quilez, I. (2010). Better Fog. [Web log post] Retrieved from http://www.iquilezles.org/www/articles/fog/fog.htm

Nguyuen, H. (2008). Chapter 13. Volumetric Light Scattering as a Post-Process. CPU Gems 3. Retrieved March 30, 2014, from http://http.developer.nvidia.com/GPUGems3/gpugems3_ch13.html